Imaging devices having piezoelectric transceivers

ABSTRACT

A micromachined ultrasonic transducer (MUT). The MUT includes: a substrate; a membrane suspending from the substrate; a bottom electrode disposed on the membrane; a piezoelectric layer disposed on the bottom electrode and an asymmetric top electrode is disposed on the piezoelectric layer. The areal density distribution of the asymmetric electrode along an axis has a plurality of local maxima, wherein locations of the plurality of local maxima coincide with locations where a plurality of anti-nodal points at a vibrational resonance frequency is located.

CROSS REFERENCE

This application is a continuation of U.S. patent application Ser. No.15/951,121, filed Apr. 11, 2018.

BACKGROUND Technical Field

The present invention relates to imaging devices, and more particularly,to imaging devices having micromachined ultrasound transducers (MUTs).

Background of the Invention

A non-intrusive imaging system for imaging internal organs of a humanbody and displaying images of the internal organs transmits signals intothe human body and receives signals reflected from the organs.Typically, transducers, such as capacitive transduction (cMUTs) orpiezoelectric transduction (pMUTs), that are used in an imaging systemare referred to as transceivers and some of the transceivers are basedon photo-acoustic or ultrasonic effects.

In general, a MUT includes two or more electrodes and the topology ofthe electrodes affects both electrical and acoustic performances of theMUT. For instance, the amplitude of acoustic pressure generated by apMUT increases as the size of the electrodes increase, to therebyimprove the acoustic performance of the pMUT. However, as the size ofthe electrodes increases, the capacitance also increases to degrade theelectrical performance of the pMUT. In another example, the amplitude ofacoustic pressure at a vibrational resonance frequency of the pMUT isaffected by the shape of the electrodes. As such, there is a need formethods for designing electrodes to enhance both acoustical andelectrical performances of the transducers.

SUMMARY OF THE DISCLOSURE

In embodiments, a micromachined ultrasonic transducer (MUT) includes anasymmetric top electrode. The areal density distribution of theasymmetric electrode along an axis has a plurality of local maxima,wherein locations of the plurality of local maxima coincide withlocations where a plurality of anti-nodal points at a vibrationalresonance frequency are located.

In embodiments, a micromachined ultrasonic transducer (MUT) includes asymmetric top electrode. The areal density distribution of the symmetricelectrode along an axis has a plurality of local maxima, whereinlocations of the plurality of local maxima coincide with locations wherea plurality of anti-nodal points at a vibrational resonance frequencyare located.

In embodiments, a transducer array includes a plurality of micromachinedultrasonic transducers (MUTs). Each of the plurality of MUTs includes anasymmetric top electrode.

In embodiments, an imaging device includes a transducer array that has aplurality of micromachined ultrasonic transducers (MUTs). Each of theplurality of MUTs includes a symmetric top electrode. The areal densitydistribution of the symmetric electrode along an axis has a plurality oflocal maxima and wherein locations of the plurality of local maximacoincide with locations where a plurality of anti-nodal points at avibrational resonance frequency are located.

BRIEF DESCRIPTION OF THE DRAWINGS

References will be made to embodiments of the invention, examples ofwhich may be illustrated in the accompanying figures. These figures areintended to be illustrative, not limiting. Although the invention isgenerally described in the context of these embodiments, it should beunderstood that it is not intended to limit the scope of the inventionto these particular embodiments.

FIG. 1 shows an imaging system according to embodiments of the presentdisclosure.

FIG. 2 shows a schematic diagram of an imager according to embodimentsof the present disclosure.

FIG. 3A shows a side view of a transceiver array according toembodiments of the present disclosure.

FIG. 3B shows a top view of a transceiver tile according to embodimentsof the present disclosure.

FIG. 4A shows a top view of a MUT according to embodiments of thepresent disclosure.

FIG. 4B shows a cross sectional view of a MUT, taken along a direction4-4 in FIG. 4A, according to embodiments of the present disclosure.

FIGS. 5A-5E show vibrational mode shapes of MUTs according toembodiments of the present disclosure.

FIG. 6A shows a plot of acoustic response of a MUT as a function offrequency according to embodiments of the present disclosure.

FIG. 6B shows a top view a MUT according to embodiments of the presentdisclosure.

FIG. 6C shows an areal density distribution of a top electrode accordingto embodiments of the present disclosure.

FIG. 7A shows a plot of acoustic response of a MUT as a function offrequency according to embodiments of the present disclosure.

FIG. 7B shows a top view a MUT according to embodiments of the presentdisclosure.

FIG. 7C shows an areal density distribution of a top electrode accordingto embodiments of the present disclosure.

FIG. 8A shows a plot of acoustic response of a MUT as a function offrequency according to embodiments of the present disclosure.

FIG. 8B shows a top view a MUT according to embodiments of the presentdisclosure.

FIG. 8C shows an areal density distribution of a top electrode accordingto embodiments of the present disclosure.

FIG. 9A shows a plot of acoustic response of a MUT as a function offrequency according to embodiments of the present disclosure.

FIG. 9B shows a top view a MUT according to embodiments of the presentdisclosure.

FIG. 9C shows an areal density distribution of a top electrode accordingto embodiments of the present disclosure.

FIG. 10A shows a plot of acoustic response of a MUT as a function offrequency according to embodiments of the present disclosure.

FIG. 10B shows a top view a MUT according to embodiments of the presentdisclosure.

FIG. 10C shows an areal density distribution of a top electrodeaccording to embodiments of the present disclosure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description, for purposes of explanation, specificdetails are set forth in order to provide an understanding of thedisclosure. It will be apparent, however, to one skilled in the art thatthe disclosure can be practiced without these details. Furthermore, oneskilled in the art will recognize that embodiments of the presentdisclosure, described below, may be implemented in a variety of ways,such as a process, an apparatus, a system, or a device.

Elements/components shown in diagrams are illustrative of exemplaryembodiments of the disclosure and are meant to avoid obscuring thedisclosure. Reference in the specification to “one embodiment,”“preferred embodiment,” “an embodiment,” or “embodiments” means that aparticular feature, structure, characteristic, or function described inconnection with the embodiment is included in at least one embodiment ofthe disclosure and may be in more than one embodiment. The appearancesof the phrases “in one embodiment,” “in an embodiment,” or “inembodiments” in various places in the specification are not necessarilyall referring to the same embodiment or embodiments. The terms“include,” “including,” “comprise,” and “comprising” shall be understoodto be open terms and any lists that follow are examples and not meant tobe limited to the listed items. Any headings used herein are fororganizational purposes only and shall not be used to limit the scope ofthe description or the claims. Furthermore, the use of certain terms invarious places in the specification is for illustration and should notbe construed as limiting.

FIG. 1 shows a schematic diagram of an imaging system 100 according toembodiments of the present disclosure. As depicted, the system 100 mayinclude: an imager 120 that generates and transmits pressure waves 122toward an internal organ 112, such as heart, in a transmit mode/processand receives pressure waves reflected from the internal organ; and adevice 102 that sends and receives signals to the imager through acommunication channel 130. In embodiments, the internal organ 112 mayreflect a portion of the pressure waves 122 toward the imager 120, andthe imager 120 may capture the reflected pressure waves and generateelectrical signals in a receive mode/process. The imager 120 maycommunicate electrical signals to the device 102 and the device 102 maydisplay images of the organ or target on a display/screen 104 using theelectrical signals.

In embodiments, the imager 120 may be used to get an image of internalorgans of an animal, too. The imager 120 may also be used to determinedirection and velocity of blood flow in arteries and veins as in Dopplermode imaging and also measure tissue stiffness. In embodiments, thepressure wave 122 may be acoustic waves that can travel through thehuman/animal body and be reflected by the internal organs, tissue orarteries and veins.

In embodiments, the imager 120 may be a portable device and communicatesignals through the communication channel 130, either wirelessly (usinga protocol, such as 802.11 protocol) or via a cable (such as USB2, USB3, USB 3.1, USB-C, and USB thunderbolt), with the device 102. Inembodiments, the device 102 may be a mobile device, such as cell phoneor iPad, or a stationary computing device that can display images to auser.

In embodiments, more than one imager may be used to develop an image ofthe target organ. For instance, the first imager may send the pressurewaves toward the target organ while the second imager may receive thepressure waves reflected from the target organ and develop electricalcharges in response to the received waves.

FIG. 2 shows a schematic diagram of the imager 120 according toembodiments of the present disclosure. In embodiments, the imager 120may be an ultrasonic imager. As depicted in FIG. 2, the imager 120 mayinclude: a transceiver tile(s) 210 for transmitting and receivingpressure waves; a coating layer(s) 212 that operate as a lens forsetting the propagation direction of and/or focusing the pressure wavesand also functions as an acoustic impedance interface between thetransceiver tile and the human body 110; a control unit 202, such asASIC chip (or, shortly ASIC), for controlling the transceiver tile(s)210 and coupled to the transducer tile 210 by bumps; Field ProgrammableGate Arrays (FPGAs) 214 for controlling the components of the imager120; a circuit(s) 215, such as Analogue Front End (AFE), forprocessing/conditioning signals; an acoustic absorber layer 203 forabsorbing waves that are generated by the transducer tiles 210 andpropagate toward the circuit 215; a communication unit 208 forcommunicating data with an external device, such as the device 102,through one or more ports 216; a memory 218 for storing data; a battery206 for providing electrical power to the components of the imager; andoptionally a display 217 for displaying images of the target organs.

In embodiments, the device 102 may have a display/screen. In such acase, the display may not be included in the imager 120. In embodiments,the imager 120 may receive electrical power from the device 102 throughone of the ports 216. In such a case, the imager 120 may not include thebattery 206. It is noted that one or more of the components of theimager 120 may be combined into one integral electrical element.Likewise, each component of the imager 120 may be implemented in one ormore electrical elements.

In embodiments, the user may apply gel on the skin of the human body 110before the body 110 makes a direct contact with the coating layer 212 sothat the impedance matching at the interface between the coating layer212 and the human body 110 may be improved, i.e., the loss of thepressure wave 122 at the interface is reduced and the loss of thereflected wave travelling toward the imager 120 is also reduced at theinterface. In embodiments, the transceiver tiles 210 may be mounted on asubstrate and may be attached to an acoustic absorber layer. This layerabsorbs any ultrasonic signals that are emitted in the reversedirection, which may otherwise be reflected and interfere with thequality of the image.

As discussed below, the coating layer 212 may be only a flat matchinglayer just to maximize transmission of acoustic signals from thetransducer to the body and vice versa. Beam focus is not required inthis case, because it can be electronically implemented in control unit202. The imager 120 may use the reflected signal to create an image ofthe organ 112 and results may be displayed on a screen in a variety offormat, such as graphs, plots, and statistics shown with or without theimages of the organ 112.

In embodiments, the control unit 202, such as ASIC, may be assembled asone unit together with the transceiver tiles. In other embodiments, thecontrol unit 202 may be located outside the imager 120 and electricallycoupled to the transceiver tile 210 via a cable. In embodiments, theimager 120 may include a housing that encloses the components 202-215and a heat dissipation mechanism for dissipating heat energy generatedby the components.

FIG. 3A shows a side view of a transceiver array 200 according toembodiments of the present disclosure. FIG. 3B shows a top view of atransceiver tile 210 according to embodiments of the present disclosure.In embodiments, the array 200 may include one or more transceiver tiles210. As depicted, the transceiver array 200 may include one or moretransceiver tiles 210 arranged in a predetermined manner. For instance,as depicted in FIG. 3A, the transceiver tiles (or, shortly tiles) 210may be physically bent to further form a curved transceiver array anddisposed in the imager 120. It should be apparent to those of ordinaryskill in the art that the imager 120 may include any suitable number oftiles and the tiles may be arranged in any suitable manner, and eachtile 210 may include any suitable number of piezoelectric elements 302that are disposed on a transceiver substrate 304. On the substrate 304,one or multiple number of temperature sensors 320 may be placed in orderto monitor the temperature of the transceiver tile 210 during operation.In embodiments, the transceiver array 200 may be a micro-machined arrayfabricated from a substrate.

FIG. 4A shows a top view of a MUT 400 according to embodiments of thepresent disclosure. FIG. 4B shows a cross-sectional view of the MUT 400in FIG. 4A, taken along the line 4-4, according to embodiments of thepresent disclosure. As depicted, the MUT may include: a membrane layer406 suspended from a substrate 402; a bottom electrode (O) 408 disposedon the membrane layer (or, shortly membrane) 406; a piezoelectric layer410 disposed on the bottom electrode (O) 408; and a top electrode (X)412 disposed on the piezoelectric layer 410.

In embodiments, the substrate 402 and the membrane 406 may be onemonolithic body and the cavity 404 may be formed to define the membrane406. In embodiments, the cavity 404 may be filled with a gas at apredetermined pressure or an acoustic damping material to control thevibration of the membrane 406. In embodiments, the geometrical shape ofthe projection area of the top electrode 412 may be configured tocontrol the dynamic performance and capacitance magnitude of the pMUT400.

In embodiments, each MUT 400 may by a pMUT and include a piezoelectriclayer formed of at least one of PZT, KNN, PZT-N, PMN-Pt, AlN, Sc—AlN,ZnO, PVDF, and LiNiO₃. In alternative embodiments, each MUT 400 may be acMUT. In FIG. 4A, each MUT 400 is shown to have a rectangular shape. Inembodiments, each MUT may include a top electrode that has an ellipticalshape when viewed from the top of the MUT 400. Hereinafter, the termshape of the top electrode 412 refers to a two-dimensional shape of thetop electrode obtained by projecting the top electrode on to the x-yplane. Also, the shape of the top electrode is called symmetric if theshape is symmetric with respect to the two lines 450 and 452, where thelines 450 and 452 are parallel to the x- and y-axes, respectively, andpass through the midpoint of the top electrode on the x-axis. Also,hereinafter, the x-axis extends along the direction where the topelectrode has the longest dimension. It should be apparent to those ofordinary skill in the art that the top electrode may have other suitablesymmetric shape, such as, square, circle, rectangle, and oval, so on.

FIGS. 5A-5E show five vibrational modes 500, 510, 520, 530, and 540,according to embodiments of the present disclosure. In FIGS. 5A-5E, eachof the MUTs 502, 512, 522, 532, and 542 is represented by a single linefor the purpose of illustration, where each single line shows thecurvature of the stack of layers in a MUT. During operation, the stackof layers having the membrane 406, bottom electrode 408, piezoelectriclayer 410, and top electrode 412 may move as a single body in thevertical direction, and may be deformed to have the curvature of thesingle line on the x-z plane. Also, the lines 502, 512, 522, 532, and542 that correspond to different vibrational modes show the curvaturesof the stack at different vibrational modes.

In embodiments, the five vibrational modes 500, 510, 520, 530, and 540may be associated with five vibrational resonance frequencies, f1, f2,f3, f4, and f5, respectively. In FIGS. 5A-5E, only five vibrationalmodes are shown. However, it should be apparent to those of ordinaryskill in the art that a MUT may operate in more than five vibrationalresonance modes (or shortly vibrational modes).

In FIG. 5A, the MUT 502 may operate in the first vibrational mode 500,where the arrow 504 indicates that the MUT 502 (more specifically, thestack of layers) moves in the vertical direction in the first mode 500.In embodiments, the first vibrational mode 500 may be symmetric, i.e.,the mode shape is symmetric with respect to the centerline 506 of theMUT. In embodiments, the shape of the top electrode of the MUT 502 maybe symmetric and similar to the shape of the top electrode 412.

In FIG. 5C, the MUT 522 may operate in the third vibrational mode 520.In embodiments, the third vibrational mode 520 may be symmetric, i.e.,the mode shape is symmetric with respect to the centerline 506.Hereinafter, the term symmetric vibrational mode refers to a vibrationalmode where the locations of the anti-nodal points, such as 525, 526, and527, (i.e., the peak amplitudes) are arranged symmetrically with respectto a centerline 506, and the centerline 506 represents a line that isparallel to the z-axis and passes through the midpoint of the MUT on thex-axis. Likewise, the term asymmetric vibrational mode refers to avibrational mode where the locations of the anti-nodal points, such as516 and 517 in FIG. 5B, are arranged asymmetrically with respect to thecenterline 506.

In the third vibrational mode 520, the MUT 522 may have two nodal pointsand three anti-nodal points (or equivalently, three peak amplitudepoints) 525, 526, and 527. In embodiments, the shape of the topelectrode of the MUT 522 may be symmetric and similar to the shape ofthe top electrode 412.

In FIG. 5E, the MUT 542 may operate in the fifth vibrational mode 540.In embodiments, the fifth vibrational mode 540 may be symmetric, i.e.,the mode shape is symmetric with respect to the centerline 506. In thefifth vibrational mode, the MUT 542 may have four nodal points and fiveanti-nodal points (i.e., five peak amplitude points) 544, 545, 546, 547,and 548. In embodiments, the shape of the top electrode of the MUT 542may be symmetric and similar to the shape of the top electrode 412.

In embodiments, if the top electrode has a symmetric shape, the MUTs mayoperate in the symmetric vibrational modes 500, 520 and 540. Inembodiments, the geometrical shape of the top electrode may be changedso that the MUT may vibrate in one or more asymmetric vibrational modesas well as symmetric vibrational modes. In FIG. 5B, the MUT 512 mayoperate in an asymmetric second vibrational mode 510. In the asymmetricsecond vibrational mode, the MUT 512 may have one nodal point and twoanti-nodal points (or equivalently, two peak amplitude points) 516 and517. The shape of the top electrode corresponding to the MUT 512 isdescribed in conjunction with FIGS. 7A-7C.

In FIG. 5D, the MUT 532 may operate in an asymmetric third vibrationalmode 530. As depicted, the vibrational mode 530 may be asymmetric withrespect to the centerline 506. In the asymmetric third vibrational mode,the MUT 532 may have two nodal points and three anti-nodal points (orequivalently, three peak amplitude points) 534, 535, and 537. Inembodiments, the peak amplitude 539 of the asymmetric third vibrationalmode 530 may be higher than the peal amplitude 529 of the symmetricthird vibrational mode 520. In general, an asymmetric vibrational mode(such as 530) may have higher peak amplitude than a symmetricvibrational mode of the same order (such as 520).

In general, the acoustic pressure performance, which refers to theenergy of an acoustic pressure wave generated by each MUT at afrequency, may increase as the peak amplitude of the MUT increases atthe frequency. As depicted in FIGS. 5C and 5D, an asymmetric vibrationalmode may have higher peak amplitude than a symmetric vibrational mode ofthe same order. As such, a MUT operating in an asymmetric vibrationalmode may yield higher acoustic pressure performance than a MUT operatingin a symmetric vibrational mode of the same order. Also, the frequencyof a symmetric vibrational mode may be different from the frequency ofan asymmetric vibration mode of the same order. As such, in embodiments,the vibrational resonance frequency of each MUT can be tuned byswitching from symmetric mode to asymmetric mode of the same order (orvice versa).

FIG. 6A shows a plot 600 of acoustic response of a MUT 620 having a topelectrode 622 according to embodiments of the present disclosure. FIG.6B shows a top view of the MUT 620 according to embodiments of thepresent disclosure. In FIG. 6B, the height, H 641 represents thevertical dimension of the top electrode, and FIG. 6C shows adistribution 660 of the height H 641 (i.e., areal density distribution)along the x-axis 642 according to embodiments of the present disclosure.For the purpose of illustration, the vibrational mode 500 in FIG. 5A isshown over the plot 660. As depicted, the projection area of the topelectrode 622 has an elliptical shape, where the elliptical shape issymmetric with respect to both the centerline 630 and the x-axis 642,i.e., the shape of the top electrode is symmetric. As such, the MUT 620may have strong acoustic response at the symmetric vibrational modes f1,f3 and f5. Also, the MUT 620 may have very weak acoustic response at theasymmetric vibrational modes 510 (f2) and 530 (f4), as indicated by thecircles 612 and 614.

In embodiments, the location 625 where the height H 641 is maximum isthe same as the location 503 where the anti-nodal point (i.e., peakamplitude) of the vibrational mode 500 occurs. As a consequence, the MUT620 may have strongest acoustic response at the frequency f1, asindicated by the circle 602, where f1 corresponds to the first symmetricvibrational mode (500).

FIG. 7A shows a plot 700 of acoustic response of a MUT 720 having a topelectrode 722 according to embodiments of the present disclosure. FIG.7B shows a top view of the MUT 720 according to embodiments of thepresent disclosure. In FIG. 7B, the height, H 741 represents thevertical dimension of the top electrode. FIG. 7C shows a distribution760 of the height H 741 (or, equivalently areal density distribution)along the x-axis 742 according to embodiments of the present disclosure.For the purpose of illustration, the vibrational mode 510 in FIG. 5B isshown over the areal distribution plot 760. As discussed above, theterms height and areal density are used interchangeably since the heightdistribution 760 may represent the distribution of the areal density ofthe top electrode 722 along the x-axis 742.

As depicted in FIG. 7B, the shape of the top electrode 722 may beasymmetric since the top electrode 722 is not symmetric with respect tothe centerline 730 that passes through the midpoint of the top electrodeon the x-axis. As a consequence, the MUT 720 may operate in bothsymmetric vibrational modes (f1, f3, and f5) and asymmetric vibrationalmodes (f2 and f4). In embodiments, the locations 726 and 728 of thelocal maxima H1 735 and H2 737 of the areal density distribution 760coincide with the anti-nodal points 516 and 517 of the vibrational mode510, respectively. As a consequence, the MUT 720 may have the strongestacoustic response at the frequency f2, as indicated by a circle 702.

In embodiments, the ratio of L2 733 to L1 731 may be adjusted to controlthe locations 726 and 728 of the local maxima of the areal densitydistribution. For instance, the ratio of L2 733 to L1 731 may be greaterthan 1.05. In embodiments, the ratio of H1 735 to H2 737 may be adjustedto control the acoustic response at the frequency f2. For instance, theratio of heights H1 735 to H2 737 may be greater than 1.05.

In embodiments, the distribution of areal density 760 of the topelectrode 722 may affect the acoustic response of the MUT 720. Asdescribed in conjunction with FIGS. 9A-9C, the areal densitydistribution of the top electrode 722 may be modified so that theacoustic response has the maximum value at the frequency f4.

FIG. 8A shows a plot 800 of acoustic response of a MUT 820 having a topelectrode 822 according to embodiments of the present disclosure. FIG.8B shows a top view of the MUT 820 according to embodiments of thepresent disclosure. FIG. 8C shows the distribution 860 of the height H841 (or, equivalently distribution of areal density) of the topelectrode 822 along the x-axis 843. For the purpose of illustration, thesymmetric vibrational mode 520 in FIG. 5C is also shown over the plot860. As depicted, the top electrode 822 may be symmetric since it issymmetric with respect to both the x-axis 843 and the centerline 830,and as a consequence, the MUT 820 may have strong acoustic response atthe symmetric vibrational modes (f1, f3, and f5), and very weak acousticresponse at the asymmetric vibrational modes (f2 and f4) as indicated bycircles 804 and 806.

As depicted, the areal density distribution 860 may have local maxima atthree locations 824, 825, and 826. Also, these three locations 824, 825,and 826 respectively coincide with the locations 525, 526, and 527 wherethe anti-nodal points of the third symmetric vibrational mode 520 arelocated. As a consequence, the MUT 820 may have the strongest acousticresponse at the frequency f3, as indicated by the circle 802.

In embodiments, the ratio of L3 844 to L4 846 may be adjusted to controlthe location 824 of the local maximum of the areal density distribution.For instance, the ratio of the ratio of L4 846 to L3 844 may be equal toand greater than 10. In embodiments, the ratio of H3 850 to H4 852 maybe adjusted to control the acoustic response at the frequency f2. Forinstance, the ratio of H4 852 to H3 850 may be equal to or greater than1.05.

FIG. 9A shows a plot 900 of acoustic response of a MUT 920 having a topelectrode 922 according to embodiments of the present disclosure. FIG.9B shows a top view of the MUT 920 according to embodiments of thepresent disclosure. FIG. 9C shows the distribution 960 of the height H941 along the x-axis 943. For the purpose of illustration, theasymmetric vibrational mode 530 in FIG. 5D is also shown over the height(or areal density) distribution plot 960. As depicted, the top electrode922 may be asymmetric with respect to the centerline 930, and as aconsequence, the MUT 920 may have both the symmetric vibrational modes(f1, f3, and f5) and the asymmetric vibrational modes (f2 and f4). Also,in embodiments, the acoustic response may be strongest at the asymmetricvibrational frequency f4 (530), as indicated by a circle 902.

In embodiments, the areal density distribution 960 may have local maximaat three locations 924, 925, and 926. Also, these three locations 924,925, and 926 may coincide with the locations 534, 535, and 537 where theanti-nodal points of the vibrational mode 530 are located. As aconsequence, the MUT 920 may have the strongest acoustic response at thefrequency f4, as indicated by a circle 902.

FIG. 10A shows a plot 1000 of acoustic response of a MUT 1020 having atop electrode 1022 according to embodiments of the present disclosure.FIG. 10B shows a top view of the MUT 1020 according to embodiments ofthe present disclosure. FIG. 10C shows the distribution 1060 of theheight H 1041 along the x-axis 1043. For the purpose of illustration,the symmetric vibrational mode 540 in FIG. 5E is also shown over theplot 1060. As depicted, the top electrode 1022 may be symmetric since itis symmetric with respect to both the x-axis 1043 and the centerline1030, and as a consequence, the MUT 1020 may have strong acousticresponse at the symmetric vibrational modes (f1, f3, and f5) and veryweak acoustic response at the asymmetric vibrational modes (f2 and f4).Also, the acoustic response is the strongest at the fifth symmetricvibrational mode f5, as indicated by a circle 1002.

In embodiments, the areal density distribution 1060 may have localmaxima at five locations 1024, 1025, 1026, 1027, and 1028. Also, thesefive locations 1024, 1025, 1026, 1027, and 1028 may coincide with thelocations 544, 545, 546, 547, and 548 where the peak amplitudes of thevibrational mode 540 are located. As a consequence, the MUT 1020 mayhave the strongest acoustic response at the frequency f5.

In embodiments, as described in conjunction with FIGS. 6A-10C, asymmetric top electrode may have strong acoustic response at symmetricvibrational modes and very weal acoustic response at asymmetricvibrational modes. Also, in embodiments, an asymmetric top electrode mayhave strong acoustic response at both symmetric and asymmetricvibrational modes. In embodiments, to increase the acoustic response ata vibrational mode, the areal density distribution of the top electrodemay be adjusted so that the local maxima (or maximum) of the arealdensity distribution are (is) located at the locations(s) where theanti-nodal point(s) of the vibrational mode(s) are (is) located.

For the purpose of illustration, only five vibrational modes f1-f5 areshown in FIGS. 6A-10C. However, it should be apparent to those ofordinary skill in the art that a MUT may have more than five vibrationalmodes. Also, it should be apparent to those of ordinary skill in the artthat the areal density distribution may be adjusted to control themagnitude of the acoustic response at the higher vibrational modes inthe similar manner as described in FIGS. 6A-10C.

It is noted that each of the MUTs 302 in FIG. 3B may be a piezoelectricmicromachined ultrasound transducer (pMUT). However, it should beapparent to those of ordinary skill in the art that the transceiver tile210 may include an array of capacitive micromachined ultrasoundtransducers (cMUTs), i.e., the piezoelectric elements 302 may bereplaced by cMUTs. In such a case, the top electrode of a CMUT may havea shape that is similar to one of shapes of the top electrodes 622, 722,822, 922, and 1022, so that the acoustic response of the cMUT iscontrolled at various vibrational resonance frequencies, based on theprinciples described in conjunction with FIGS. 6A-10C.

While the invention is susceptible to various modifications andalternative forms, specific examples thereof have been shown in thedrawings and are herein described in detail. It should be understood,however, that the invention is not to be limited to the particular formsdisclosed, but to the contrary, the invention is to cover allmodifications, equivalents, and alternatives falling within the scope ofthe appended claims.

What is claimed is:
 1. A micromachined ultrasonic transducer (MUT),comprising: a substrate; a first electrode coupled to the substrate,wherein the first electrode that is symmetric with respect to a firstaxis and asymmetric with respect to a second axis that is normal to thefirst axis, the first axis extending along a direction where the firstelectrode has a longest dimension, the second axis passing through amidpoint between two ends of the first electrode on the first axis; anda second electrode coupled to the substrate and non-co-planar with thefirst electrode, wherein the first and second electrodes are separatedfrom one another in a vertical direction, and wherein the firstelectrode comprises a top electrode and the second electrode comprises abottom electrode.
 2. The MUT of claim 1, wherein the MUT is a capacitivemicromachined ultrasound transducer (cMUT).
 3. The MUT of claim 1,wherein the MUT is a piezoelectric micromachined ultrasound transducer(pMUT).
 4. The MUT of claim 3, further comprising: a membrane suspendingfrom the substrate; a piezoelectric layer; wherein the first and secondelectrodes are disposed on the piezoelectric layer.
 5. The MUT of claim4, wherein the piezoelectric layer is formed of at least one of PZT,KNN, PZT-N, PMN-Pt, AIN, Sc-AIN, ZnO, PVDF, and LiNiO3.
 6. The MUT ofclaim 1, wherein the first and second electrodes are separated from oneanother along a third axis normal to both the first and second axes. 7.The MUT of claim 4, wherein the first electrode is disposed on a firstside of the piezoelectric layer and the second electrode is disposed ona second side of the piezoelectric layer facing a direction opposite thefirst side.
 8. An imaging device, comprising: a transducer arrayincluding a plurality of micromachined ultrasonic transducers (MUTs),each of the plurality of MUTs comprising: a substrate; a first electrodecoupled to the substrate, wherein the first electrode is symmetric withrespect to a first axis and asymmetric with respect to a second axisthat is normal to the first axis, the first axis extending along adirection where the first electrode has a longest dimension, the secondaxis passing through a midpoint between two ends of the first electrodeon the first axis; and a second electrode coupled to the substrate andnon-co-planar with the first electrode, wherein the first and secondelectrodes are separated from one another in a vertical direction, andwherein the first electrode comprises a top electrode and the secondelectrode comprises a bottom electrode.
 9. The imaging device of claim8, wherein each of the plurality of MUTs is a capacitive micromachinedultrasound transducer (cMUT).
 10. The imaging device of claim 8, whereineach of the plurality of MUTs is a piezoelectric micromachinedultrasound transducer (pMUT).
 11. The imaging device of claim 8, whereineach of the plurality of MUTs further comprises: a membrane suspendingfrom the substrate; a piezoelectric layer, wherein the first and secondelectrodes are disposed on the piezoelectric layer.
 12. The imagingdevice of claim 11, wherein the piezoelectric layer is formed of atleast one of PZT, KNN, PZT-N, PMN-Pt, AIN, Sc-AIN, ZnO, PVDF, andLiNiO3.
 13. The imaging device of claim 8, wherein the first and secondelectrodes are separated from one another along a third axis normal toboth the first and second axes.
 14. The imaging device of claim 11,wherein the first electrode is disposed on a first side of thepiezoelectric layer and the second electrode is disposed on a secondside of the piezoelectric layer facing a direction opposite the firstside.
 15. A micromachined ultrasonic transducer (MUT), comprising: asubstrate; a first electrode coupled to the substrate, wherein the firstelectrode that is symmetric with respect to a first axis and asymmetricwith respect to a second axis that is normal to the first axis, thefirst axis extending along a direction where the first electrode has alongest dimension, the second axis passing through a midpoint betweentwo ends of the first electrode on the first axis; a second electrodecoupled to the substrate and non-co-planar with the first electrode; anda piezoelectric layer, wherein the first and second electrodes aredisposed on the piezoelectric layer, wherein the first electrode isdisposed on a first side of the piezoelectric layer and the secondelectrode is disposed on a second side of the piezoelectric layer facinga direction opposite the first side.
 16. The MUT of claim 15, whereinthe MUT is a capacitive micromachined ultrasound transducer (cMUT). 17.The MUT of claim 15, wherein the MUT is a piezoelectric micromachinedultrasound transducer (pMUT).
 18. The MUT of claim 15, furthercomprising a membrane suspending from the substrate.
 19. The MUT ofclaim 15, wherein the piezoelectric layer is formed of at least one ofPZT, KNN, PZT-N, PMN-Pt, AIN, Sc-AIN, ZnO, PVDF, and LiNiO3.
 20. The MUTof claim 15, wherein the first and second electrodes are separated fromone another along a third axis normal to both the first and second axes.21. An imaging device, comprising: a transducer array including aplurality of micromachined ultrasonic transducers (MUTs), each of theplurality of MUTs comprising: a substrate; a first electrode coupled tothe substrate, wherein the first electrode is symmetric with respect toa first axis and asymmetric with respect to a second axis that is normalto the first axis, the first axis extending along a direction where thefirst electrode has a longest dimension, the second axis passing througha midpoint between two ends of the first electrode on the first axis; asecond electrode coupled to the substrate and non-co-planar with thefirst electrode; and a piezoelectric layer, wherein the first and secondelectrodes are disposed on the piezoelectric layer, wherein the firstelectrode is disposed on a first side of the piezoelectric layer and thesecond electrode is disposed on a second side of the piezoelectric layerfacing a direction opposite the first side.
 22. The imaging device ofclaim 21, wherein each of the plurality of MUTs is a capacitivemicromachined ultrasound transducer (cMUT).
 23. The imaging device ofclaim 21, wherein each of the plurality of MUTs is a piezoelectricmicromachined ultrasound transducer (pMUT).
 24. The imaging device ofclaim 21, wherein each of the plurality of MUTs further comprises amembrane suspending from the substrate.
 25. The imaging device of claim21, wherein the piezoelectric layer is formed of at least one of PZT,KNN, PZT-N, PMN-Pt, AIN, Sc-AIN, ZnO, PVDF, and LiNiO3.
 26. The imagingdevice of claim 21, wherein the first and second electrodes areseparated from one another along a third axis normal to both the firstand second axes.